The known electrophotographic methods for obtaining a picture on films are based on transfer of a latent image from a charged and exposed selenium layer onto a dielectric film by producing a gas discharge in the air gap between the selenium layer and the dielectric film (U.S. Pat. No. 2,937,943, published on May 24, 1960).
This method has insufficient resolution. The picture thus obtained is blurred due to scattering of ions in the gas-discharge gap.
Furthermore, when developing the wide fields on the transferred latent image a marginal effect is observed, as well as on the exposed selenium layer. The value of the charge of the transferred image is different for the image elements of different width. Therefore, during the development the optical density of the image elements is different.
Known in the art is a charge-free method for obtaining an image on a dielectric film, in which the dielectric film is brought in contact with surface of a charged and exposed electrophotographic layer, then the field of the latent image is developed and fixed (cf. R. Shaffert "Electrophotography", "Mir" publishers, Moscow, 1968, p.p. 323-333).
In this discharge-free method the image is sharper than in the above-mentioned method, however, it is still inadequate for practical purposes. This is associated with a decrease in the gradient of the field intensity at the boundaries of the fine lines on the film surface to the refraction of the field line in the film.
Like in the methods based on a gas discharge, the discharge-free method does not suitable for producing an image with wide fields without fringe effect. Furthermore, the optical density of the image elements of different width is also not constant.
All this hinders utilization of the images obtained for making copies on the light-sensitive layer of an offset plate. Furthermore, the above described method does not allow one to obtain several images using a once-exposed electrophotographic layer.